At optical wavelengths, emission from the shell of a shell-type remnant can be seen in one of two ways. If there is at least partially neutral material present ahead of the shock (often taken as an indication that the object is the remnant of a Type Ia SN; see [1]}), then charge exchange can take place in the post-shock environment giving rise to a strong H\(\alpha \)  component [2]}, as shown in the middle panel of Figure REF . The temperatures in these remnants is too hot to allow recombination lines in other elements. In older remnants where the temperatures have cooled significantly, recombination lines from various elements begin to dominate (see Section ).
